Pairs of cylinders are frequently employed as tools for guiding webs of material, or for processing their surfaces. The cylinders are rotatably arranged with pivotable shafts and delimit a gap through which the web of material runs. Along a clamping line which is parallel with the shafts, the web is subjected to a pressure from a cylinder, which pressure exerts a guiding pressure or effect on the web of material, or performs web processing. This pressure must be evenly distributed over the length of the clamping line to assure that the processing is even over the width of the web and, with guiding rollers, to prevent irregularities of any slippage occurring between the rollers and the web over the width of the web, which irregularities can lead to a deformation of the web per se. Such a deformation can be the source of indexing errors when printing on the web.
An important reason for the occurrence of irregularities in the print distribution along the clamping line is the inherent deformation of the rollers because of their own weight. It is known, for example, that the forme cylinders for rotogravure printing, in particular forme cylinders of a great width, of an order of magnitude between 1.5 m to 4 m, have a tendency to sag under their own weight. Because of this cylinder sag, the pressure along the clamping line between such a forme cylinder and a counter-pressure cylinder, which is arranged above it, is reduced toward the center of the paper web. For this reason, the counter-pressure cylinder of known rotogravure printing presses is also bent to match the outer shape of the counter-pressure cylinder to the bending of the forme cylinder, and to distribute the pressure between the two cylinders evenly over the clamping line.
A counter-pressure cylinder for a rotogravure press is known from DE 30 33 320 C2, and whose shell is received, rotatably seated, in rolling bearings in the area of its ends, in adjustable bearing end plates. An actuating member, which is supported on the associated adjustable bearing end plate, and which can be actuated in the radial direction with respect to the shaft, acts on the ends of the cylinder shaft, which shaft extends through the shell and protrudes from the shell. The shell of the counter-pressure cylinder is bent by operation of the actuating member, and the counter-pressure cylinder exterior shape is matched to the shape of a forme cylinder, which has been placed against it.
A counter-pressure cylinder, which cooperates with a forme cylinder in a rotogravure printing press, is also known from DE 100 23 205 A1. A variable matching of the counter-pressure cylinder to the forme cylinder is achieved with this counter-pressure cylinder. A linear drive mechanism, which is located at each of the ends of the counter-pressure cylinder, and between a fixed shaft and a rotating shell, acts, in a vertical radial direction, downwardly on an inner ring of a rolling bearing, while the center area of the shells is maintained rotatably, but not displaceably, on the shaft.
DE 88 08 352 U1 discloses a cylinder, whose bending can be adjusted in two planes.
U.S. Pat. No. 3,638,292 and EP 0 741 253 A2 show contact pressure rollers, which have wheels in their interior, and which can be charged with a pressure medium. These wheels are arranged on a common shaft.
U.S. Pat. Nos. 4,455,727 and 3,389,450 disclose rollers, which can be bent in two planes that are offset by 90°. Actuating elements are arranged in the interior of the rollers.
A cylinder with an assembly for generating an inner tension of the cylinder, and with a control unit/regulator for controlling the assembly, and with vibration sensors, is known from DE 199 63 945 C1. The assembly and actuating members are controlled in response to the vibrations detected by the vibration sensor.
A counter-pressure cylinder is also known from U.S. Pat. No. 4,913,051, which counter-pressure cylinder consists of a shaft and of a shell which can be rotated around the shaft. Inflatable chambers are provided between the shaft and the shell of this counter-pressure cylinder. The chambers will expand or extend, after being charged with pressure, and will thus cause bending of the shell.
EP 0 331 870 A2 discloses an arrangement for the seating of cylinders. Journals of a cylinder are seated in two bearings that are arranged side-by-side in the axial direction of the cylinder. The bearings can be individually moved perpendicularly with respect to the axis of rotation by the use of pressure medium cylinders in order to compensate for bending, for example.
An exact guidance of a web, in a manner which is free of indexing errors, is made difficult, particularly in connection with rotogravure printing presses of great width, because it is extremely difficult to produce forme cylinders, also of great length, and which have an exactly constant diameter over their length. In most cases, such a long forme cylinder is slightly thicker in its center than it is at its edges or ends. A traction force, which is exerted between the forme cylinder and a counter-pressure cylinder, on a web passed through between them, is therefore typically greater in the center of the web than it is at the edges of the web.
As a result of this uneven traction force, an uneven tension profile is generated within the paper web over its width. Since, in the course of the paper webs being processed in such a press, the paper webs absorb moisture, their stretching ability increases, so that an uneven stretching of the web, in accordance with the uneven tension profile, can occur. The result can be indexing errors.
Indexing errors between the center of the web, and an edge of the web, can be compensated for with the aid of an inlet roller, which is arranged staggered between two pressure gaps. However, in this case, it is disadvantageous that, on the other side of the paper web, the indexing errors become even greater, and that there is a danger of a lateral drift-off of the paper web.
The present invention creates a symmetrical tension profile in the web of material, which symmetrical tension profile increases toward either the center or toward the web edge areas and, in the areas of high tension, creates a change in web length, in the elastic range of the paper web. In this way, the invention provides the possibility of adjusting the image points of the different colors to be imprinted on the web, without letting the paper web drift off toward one side.